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Objective Estrogen deficiency after menopause results in rapid bone loss, predisposing women to osteoporotic fractures. Genistein, a phytoestrogen present in high concentrations in soy, is an ingredient in dietary supplements aggressively marketed for bone health. However, in a recent long-duration clinical trial in postmenopausal women, the efficacy of soy extracts in reducing bone loss was disappointing. To better understand the failure of soy extracts to consistently induce a robust skeletal response in women, we investigated the long-term (5 mo) efficacy of genistein, administered as a daily oral supplement, (1) in preventing cancellous bone loss in skeletally mature virgin Long-Evans rats ovariectomized at 7 months of age and (2) in improving cancellous bone mass and architecture in aged retired-breeder rats ovariectomized at 16 or 22 months of age.

Methods Rats within each age group were randomly assigned into one of three treatment groups (n = 7-12 rats/group): (1) vehicle control, (2) genistein 485 μg/day, or (3) genistein 970 μg/day, resulting in mean (SE) serum genistein levels of 0.18 (0.10), 0.76 (0.15), and 1.48 (0.31) μM, respectively. Total tibia bone mass and density were evaluated using dual-energy x-ray absorptiometry, whereas cancellous bone mass and architecture in the tibial metaphysis, as well as cortical bone mass and architecture in the tibial diaphysis, were evaluated by micro-CT.

Results Oral genistein administered as a dietary supplement did not influence the cumulative effects of ovariectomy, aging, and/or reproductive history on cancellous and cortical bone mass and architecture.

Conclusions Serum levels of genistein similar to those in women consuming a high-soy diet are ineffective in preventing or treating bone loss in rat models for postmenopausal osteoporosis.

From the 1Skeletal Biology Laboratory, School of Biological and Population Health Sciences, Oregon State University, Corvallis, OR; 2Department of Food Science and Human Nutrition, University of Illinois at Urbana-Champaign, Urbana, IL; 3Biostatistics, School of Biological and Population Health Sciences, Oregon State University, Corvallis, OR; and 4Neuroscience Program and Department of Comparative Biosciences, University of Illinois at Urbana-Champaign, Urbana, IL.

Received July 18, 2012; revised and accepted November 8, 2012.

Funding/support: This work was supported by National Institutes of Health grant AT006268.